Method and device for radioactive decontamination of a steel wall

ABSTRACT

Disclosed a method for radioactive decontamination of a steel surface, which is performed by bringing the surface to be decontaminated into contact with a pickling solution comprising a first iron oxidation agent, wherein bringing into contact being achieved by means of the direct continuous introduction into the pickling solution of a gas comprising a second oxidation agent, and a device for implementing the said method.

BACKGROUND ART

The present invention relates to a method for the radioactivedecontamination of a steel surface, and particularly to a method for theradioactive decontamination of a steel surface using pickling. Theinvention relates, for example, to the radioactive decontamination of aninternal circuit, a metal surface, piping or item of equipment in areprocessing plant for irradiated nuclear fuel, hereinafter referred toas the “surface”. The invention relates particularly to the radioactivedecontamination of a surface made of austenitic steel, which is used toconstruct most surfaces of such plants.

The present invention also relates to a device for implementing the saidmethod.

The radioactive contamination fixed on the surfaces of reprocessingplants is mainly due to surface adsorption. This contamination includesmetastable radioactive contamination composed of Pu²⁴¹, Am, U,^(242, 244)Cm, ¹³⁷Cs, ⁹⁰Sr and particulate radioactive contaminationwith ruthenium and the following insoluble compounds: cesiumphosphomolybdate, zirconium phosphate, zirconium molybdate, plutoniumphosphate mixed molybdate of zirconium and plutonium, oxides of Mo, Sb,Al, Fe, colloidal plutonium oxides, etc.

It is therefore unnecessary to cause significant erosion todecontaminate the surfaces. It is generally accepted that, depending onthe surface, erosion of the order of between 2 and 10 μm is sufficient.

Generally speaking, and particularly in the above example ofdecontamination of a reprocessing plant for irradiated nuclear fuel,radioactive decontamination may involve two major stages:

a first stage aimed at eliminating metastable contamination and the maindeposits adhering to the surfaces, and

a second stage aimed at eliminating both particulate contamination fixedon the surfaces and residual deposits.

The first stage is a rinsing stage using a variety of sequences ofrinsing that are non-corrosive for the surface; the second state is anerosive stage that uses reagents that are corrosive for the surface andmainly consist of oxidizing mixtures such as nitric acid/fluorhydricacid, nitric acid/cerium IV nitrate, or mixtures comprising chromicacid, nitric acid and cerium nitrate.

The nitric acid/fluorhydric acid mixture has the advantage of attackingrefractory contamination deposits such as a variety of molybdates orphosphates such as Zr⁴⁺, MoO₂ ²⁺, Pu⁴⁺ and antimony oxides.

During the corrosion stage the best decontamination factors aregenerally achieved when erosion is both slow, i.e. at a kinetic rate ofthe order of 1 μm/h or less, and regular. This is fairly difficult toachieve when the circuits to be decontaminated are complex and havezones that are more sensitive to, corrosion such as, for example, weldzones, zones of mechanical stress, etc.

Furthermore, the oxidant used for the corrosion must not be too harsh.This is why the oxidant mixture of nitric acid/fluorhydric acid cannotalways be used as it is difficult to control, particularly on largeextended surfaces.

Furthermore, some surfaces of the internal circuits of reprocessingplants for irradiated nuclear fuels are made of austenitic steel. It istherefore necessary on these surfaces to use an oxidant that does notcause intergranular corrosion of the steel. The oxidation-reductioncouple of Ce^(IV)/Ce^(III) is one of the rare oxidant agents that can beused to produce a given degree of erosion on austenitic steel surfaceswithout causing excessive intergranular corrosion.

However, the methods of the prior art using the aboveoxidation-reduction couple have the particular drawback of requiring, asa precondition of effective decontamination, large quantities of cerium(of the order 0.5 mole/m²/μm⁻¹) and hence raising the price of effluenttreatment as high concentrations of cerium are not authorized in thevitreous containment matrices. The concentration of cerium required inthese methods is therefore a limiting factor.

In the methods of the prior art, the concentration of Ce^(IV) dropsthroughout the reaction, implying changing kinetics of corrosion:corrosion is too strong at the beginning and too weak at the end. Thisdrawback is overcome by the present invention since the concentration ofCe^(IV), and therefore the kinetics, is virtually constant.

In addition, the methods of the prior art, in particular the methoddescribed in the above-mentioned document, are designed to treat oxidesand not the metal surfaces of valency 0 such as austenitic steelsurfaces. The nature of the alloy to be eroded or corroded which, in areprocessing plant is exclusively austenitic steel and not INCONEL™ orINCOLOY™ or similar means that the presence of chromic acid in thedecontaminating solutions of the methods described for the latter isundesirable.

For example, patent application EP-A-0 174 317 discloses a method fordecontaminating chrome oxides on the surface of INCONEL™ steamgenerators in power plants. In this patent the Ce^(IV) used as anoxidation agent is used as an agent for regenerating Cerium IV from theCerium III formed during oxidation. The method discloses the use as acorrosion fluid of an aqueous solution of nitric acid, chromic acid andCerium nitrate in which ozone has been dissolved. It also uses agas-liquid contactor to put the ozone into solution.

In the decontamination of a steam generator according to the methoddisclosed in EP-0 174 317, a regeneration chamber for Cerium IV iscoupled with the steam generator, said chamber regenerating the CeriumIV by injecting ozone to saturation point. This cannot be envisaged in areprocessing plant due to the high α, β and γ radiochemical activity ofthe components to be decontaminated and the great complexity and varietyof components to be decontaminated.

Furthermore, the method disclosed in application EP-0 174 317 reduceswithout eliminating the variability in corrosion speed by virtue of itsdirect dependence on the value of the concentration of Cerium IV whichchanges during the course of the reaction since Cerium IV is regeneratedin batches.

DISCLOSURE OF THE INVENTION

The present invention overcomes the drawbacks described above byproviding a method for the decontamination of a steel surface consistingin bringing the surface to be decontaminated into contact with apickling solution comprising nitric acid and a first iron oxidationagent at a suitable temperature such that the face of the said surfaceis eroded by the oxidation of the metallic constituents such as Fe0,Cr0, Ni0, Mn0 it contains, said bringing into contact being achieved bymeans of the direct, continuous introduction into the pickling solutionof a gas comprising a second oxidation agent such as continuously tooxidize, at least partly, said first oxidation agent reduced by theoxidation of the metallic constituents of the steel.

During the course of the method of the invention the first agent foroxidizing the metallic constituents of the steel is reduced when itoxidizes the metallic constituents of the steel surface and it iscontinuously regenerated by the second oxidation agent, by oxidation.The first oxidation agent is therefore selected such as to be capable ofoxidizing the metallic constituents of the steel surface and the secondoxidation agent is selected as being capable of oxidizing the firstoxidation agent reduced by oxidation of the metallic constituents of thesteel to regenerate it.

The first oxidation agent may, for example, be selected from Ce^(IV),Ag2+, etc.

The gas constituting the second oxidation agent may be selected fromgases comprising ozone.

Advantageously, according to one embodiment of the method of the presentinvention, the first oxidizing agent may be cerium IV, for example ascerium IV nitrate, and the second oxidizing agent may be a gas includingozone.

According to this embodiment, the present invention makes it possible inparticular to eliminate the above drawbacks relating in particular tothe methods of the prior art using cerium.

The present invention proposes means for continuously regeneratingcerium IV throughout the reaction at a constant speed. Throughout thereaction the concentration of cerium IV is thus constantly at theoptimal value for decontamination. The present invention discloses amethod by which a second oxidizing agent, for example ozone, isdissolved in the pickling solution (also referred to below as thedecontamination solution) directly in the component to bedecontaminated. Furthermore, the method disclosed in the presentinvention makes it possible to solubilize the second oxidizing agent,for example ozone, in the pickling or decontamination solution withoutmaking any modification or adding any special fittings to theinstallation to be decontaminated.

According to the invention the pickling solution may be an aqueoussolution containing nitric acid at a concentration of approximately 0.5to 5 mol.1⁻¹ and cerium nitrate at a concentration of approximately0.001 to 0.1 mol.1⁻¹.

According to the invention the gas including ozone may also include atleast one gas selected from either oxygen and nitrogen.

According to the invention the gas including ozone may includeapproximately 1 to 20% ozone.

According to the invention the appropriate temperature may beapproximately 10 to 80° C., for example ambient temperature, for example25° C.

According to the invention the steel surface may be a surface of aninternal circuit of a reprocessing plant for irradiated nuclear fuel,for example a surface made of austenitic steel.

The ozone introduced into the pickling solution may, for example, bedissolved in the solution by using two modifications of a component thatis very common in reprocessing installations and usually used toinitiate movement in liquid solutions. In the application of the methodof the present invention the introduction of ozone into the erosionsolution may be achieved using two gas-liquid contact components thatmay either be transfer air-lifts, of either the naturally submerging orvacuum submerging type, and/or air-lift agitators. However, in theabsence of these two systems the invention may be embodied by injectinggas by means of any type of submersible piping for introducing a liquidor gaseous reagent. By virtue of their use as disclosed in the presentinvention the said devices play a dual role during decontamination:

by performing the agitating function they ensure continuous renewal ofthe liquid-solid interface at which the oxidation stages of the metalsupport and dissolution of oxidized species occur,

by providing close gas-liquid contact they ensure transfer of the secondoxidizing agent, for example ozone, into the pickling solution at thethreshold of which the regeneration stage of cerium IV occurs.

According to the method of the invention, by introducing or injectinggas containing the second oxidizing agent, for example ozone, via thetwo air-lift systems into the decontamination solution, or picklingsolution, the gas, for example ozone, plays a dual role:

it is used as a power source for hydrodynamic agitation,

it adds the second oxidizing agent, for example ozone, which isnecessary for the regeneration of cerium IV, to the solution.

The present invention also provides a device for the radioactivedecontamination of a steel surface of an internal circuit of areprocessing plant for irradiated nuclear fuel according to the methodof the present invention, said internal circuit being provided with anagitator and/or transfer air-lift, a device, in which the air-lift isused both as a means for introducing the gas containing the secondoxidizing agent into the pickling solution containing the firstoxidizing agent, and as a means for homogenizing the pickling solutionwhen the said solution is brought into contact with the steel surface tobe decontaminated.

According to the invention the device may, also include an assembly forproducing the second oxidizing agent, for example when the firstoxidizing agent is cerium IV the second oxidizing agent may be ozone andthe said production assembly may therefore be an assembly for producingozone. This assembly may be a standard type of assembly known to thoseskilled in the art for the production of ozone.

The fact that the present invention uses two different types ofair-lift, the method makes it possible to select accurately and simplywhat portion of an installation is to be decontaminated.

Other and advantages of the present invention will emerge,from thefollowing description of non-limitative examples given with reference tothe attached figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an example of an embodiment of an assembly for producing andmonitoring the concentration of ozone in the method of the invention;

FIG. 2 is a diagram showing the method of the present invention asapplied to a tank in a reprocessing plant for irradiated nuclear fuel.

The advantages, of the method of the present invention are many:

Because ozone is injected remotely into the component beingdecontaminated, the method is completely independent and therefore noaction is required on the decontaminating solution acting on a “process”component. Control of the method is limited to possible analyticalmonitoring of the concentration of corrosion elements; this may be doneby using existing sampling systems to take samples of the picklingsolution.

In practical terms, given the low quantities of ozone required, theozone production system is very small in size, it is light and mayeasily be mounted on a movable support. The ozone production system issupplied either from the compressed air available in the installation,in which case it may be necessary to provide devices for de-oiling anddrying the compressed air, or using a cylinder of reconstituted air, oragain pure oxygen. The device must be fitted with means for monitoringthe concentration of ozone output by the ozone generator in order tocontrol the proper progress of the CeIV regeneration procedure.

The attached diagram of FIG. 1 shows an assembly for producing andmonitoring concentrations of ozone in the gaseous mixture injected intothe pickling solution contained in the component to be decontaminated.In this figure reference 1 is an ozone production device, FIG. 3 asupply of oxygen or reconstituted air, reference 5 the control valves,reference 7 control pressure gauges, reference 9 a standard ozonegenerator, reference 11 means for adjusting the flow-rate of gasinjected, reference 13 an analyzer and reference 15 a pipe feeding gascontaining ozone into the pickling solution contained in the componentto be decontaminated. The same ozone production device may thus be usedsuccessively for all the components to be decontaminated.

The fact of injecting directly using existing piping means that thedirection in which the fluids flow remains within the containmentbarriers already present in the method. Since the ozone productiondevice is in overpressure relative to the component to bedecontaminated, there is no risk of the containment barriers beingbreached. The excess ozone immediately passes into the existing ventingcircuits. Given the low mass flow of ozone and the dilution by theventilation system, the method requires no device for neutralizing theexcess ozone.

Chemical Aspects of the Method of the Invention

An overview of the chemical reactions in play may be written as follows:

3Ce⁺⁴+Fe⁰⇄3Ce⁺³+Fe³⁺

6Ce⁺⁴+Cr⁰⇄6Ce⁺³+Cr⁶⁺

2Ce⁺⁴+Ni⁰⇄2Ce⁺³+Ni²⁺

7Ce⁺⁴+Mn⁰⇄7Ce⁺³+Mn⁷⁺

2H⁺+2Ce⁺³+O₃⇄2Ce⁺⁴+H₂O+O₂

Each component of the reaction mixture plays a clearly defined role:

the nitric acid is used to make the passivation layer of the metalsurfaces permeable to the ions, thereby making the stainless steelsensitive to corrosion;

the CEIV causes the corrosion and exchanges electrons with the elementsmaking up the alloy to be corroded;

the ozone is the agent of regeneration of the CeIV since theoxidation-reduction potential of the couple O3, H⁺/H₂O, O₂ is greaterthan that of the couple Ce^(IV)/Ce^(III): 2.075 V/ENH compared with 1.72V/ENH.

A key parameter in controlling corrosion speed is the concentration ofCeIV. Thus, depending on the thickness of corrosion required, whichdepends on the depth of contamination (usually somewhere between 2 and10 μm) and the ratio between the surface and the volume of the circuitin question, the initial concentration of CeIV will be selected tooptimize both the corrosion speed and the duration of the operation. Thepresent invention therefore makes it possible to control theconcentration of CeIV by in situ oxidation of the CeIII formed duringcorrosion by ozone dissolved in the pickling solution by bubbling.

Using a given total cerium nitrate concentration, the equilibriumbetween Ce^(IV) and Ce^(III) is reached as soon as the Ce^(IV)consumption and production speeds are stationary; the lower the quantityof cerium, the faster this stationary status is reached. Under theimplementation conditions of, the invention, given the small quantitiesof cerium used the stationary state is rapidly reached; moreover, theequilibrium achieved tends strongly towards the valency IV of ceriumwhich is significantly in the majority (over 90%) compared with ceriumIII. The use of the cerium introduced is thus optimal as virtually allthe cerium introduced is permanently in usable form. In order for thiscondition to be met, a slight excess of ozone must be produced.

The regeneration reaction of Ce^(IV) by ozone from Ce^(III) is written:

O₃+2H⁺+2e⁻⇄O₂+H₂O

2Ce⁺³⇄2Ce⁴⁺+2e⁻

or:

O₃+2H⁺+2Ce⁺³→O₂+H₂O+2Ce⁴⁺

The speed at which Ce^(IV) is regenerated from Ce^(III) is dependent ona large number of parameters. The process may be expressed using astandard Arrhenius equation of the type:

d[Ce^(IV) ]/dt=A _(app)exp(−E _(app)/RT)[O_(3dissolved)]^(a)[Ce^(III)]^(b)

where:

A_(app) and E_(app) are respectively the pre-exponential factor and theArrhenius activation energy of the reaction. These magnitudes aredescribed as apparent because in the present situation they aredependent on a number of factor such as acidity,

R the ideal gas constant,

T the thermodynamic temperature,

[O_(3dissolved)] the ozone concentration in the solution,

a the order of the reaction relative to the concentration of dissolvedozone,

[Ce^(III)] the concentration of cerium at valency III,

b the order of the reaction relative to the concentration of cerium atvalency III.

If the temperature and acidity of the medium are set, the regenerationspeed of CeIV is dictated by the concentration of ozone dissolved in thesolution. This concentration is only constant when the solution issaturated with ozone.

The speed at which the decontaminant solution becomes saturated withozone is dependent upon a number of factors such as the acidity of themedium, the temperature or the contactor mode used. It should be notedthat under the operating conditions of the method of the invention andin the absence of any ozone-consuming reaction, the ozone concentrationstabilizes after a few minutes (ten minutes at most). The speed at whichozone changes to its liquid phase does not therefore constitute alimiting factor for the regeneration speed of CeIV.

The concentration of ozone in equilibrium with a nitric solution isdependent upon a number of parameters, for example temperature(solubility diminishes with temperature), concentration of nitric acid(solubility diminishes slightly with nitric acid content). Tables I andII below give several values measured under operating conditions of themethod of the invention.

TABLE I [O₃]_(dissolved) as a function of temperature 0.5 < HNO₃ < 4 mol· 1⁻¹, [O₃]_(gas) = 35 g · m⁻³ θ/° C. [O₃]_(dissolved)/mg · 1⁻¹ 20 8.530 7.7 40 6.3

TABLE II [O₃]_(dissolved) as a function of [O₃]_(gas) 0.5 < HNO₃ < mol ·1⁻¹, θ = 20°°C. [O₃]_(gas g · m) ⁻³ [O₃]_(dissolved)/mg · 1⁻¹ 27 6.3 388.9 46 11.5 

The method described in the present invention can be used to controleach of the following parameters:

temperature between 10 and 50° C. for example, ambient temperature, forexample approximately 25° C.,

acidity between 0.5 and 4 mol.1⁻¹, for example HNO₃ 4 mol.1⁻¹,

initial concentration of cerium between for example 1.10⁻³ and 10⁻¹mol.1⁻¹, for example 10⁻² mol.1⁻¹,

flow-rate of ozonized gas suitable for the component to bedecontaminated and the type of contactor,

concentration of ozone in the gas, for example between 1 and 20% byweight.

For example, the concentration of dissolved ozone may vary between 1 and20mg.1⁻¹.

By using just enough cerium to give suitable kinematics, the residualcorrosion when the operation ends is reduced; stopping injection ofozone causes virtual immediate stoppage of the regeneration of CeIV andthe rapid stoppage of corrosion by virtue of the small quantity of CeIVpresent in the medium. Moreover, since the quantity of CeIV present onstoppage of ozone injection is low and known as such, it is very easy tomonitor final corrosion. The corrosion, speed may thus be controlledmaking analytical monitoring easier to set up using the set of samplesand regular analyses of samples of the corrosion solution.

FIG. 2 attached is a diagram showing the method of the present inventionas applied to a tank in a reprocessing plant for irradiated nuclearfuel. In this figure, reference 17 is the surface to be decontaminated,references 19, 21 and 101 are pipes supplying ozonized air or oxygen,reference AC is a supply of compressed air of air-lift 102, reference ALis a natural AL circuit and relates to a naturally submerging airlift(see reference 102), reference 23 is the pickling solution of theinvention, reference 25 a prioritized area for the solubilization ofozone, references 29 and 102 are other prioritized areas for thesolubilization of ozone, references 27 and 103 the overflow pipes of thesealing pots, references 31 and 104 a sealing pot, reference 33 a vacuumgenerating circuit, reference 105 a venting circuit, references 35 and106 the piping for returning regenerated solutions to the tank to bedecontaminated, reference C a circuit under negative pressure, referenceALI a submerged air-lift agitator, reference R a return to the initialtank and reference E the outlet to the vent. In this figure the dottedline numbered 40 shows decontamination limited to the surfaces of thetank and its fittings while dotted line 50 shows the decontamination ofall the surfaces of the circuit comprising the tank and associatedfittings such as the piping of the transfer components and the sealingpots.

The compressed air supply AC may also be used for injecting a gas orgaseous mixture. It performs the same functions as those of thecomponents reference 19 and 21.

The present invention is capable of using three types of air-lift:

a vacuum air-lift,

a submerged air-lift;

a naturally submerged air-lift.

EXAMPLES

Table III below shows examples of changes in the thickness of steelcorroded as a function of the concentration of CeIV after 12 hoursduring corrosion of a test-piece of 604L stainless steel with aSurface/Volume ratio of 10: HNO₃ 4 mol.1⁻¹, ozone flow-rate (d₀₃)=6g.h⁻¹, volume of solution 3 liters.

TABLE III Thickness of 604 L corroded in 12 hours as a function of[CeIV] HNO₃ 4 mol · 1⁻¹, d_(O3) = 6 g · h⁻¹, v = 3 l [CeIV]/mol · 1⁻¹e/μm 5 · 10⁻² 13.2  2 · 10⁻² 8.6 1 · 10⁻² 4.0 5 · 10⁻³ 0.7

As a comparison, table IV below shows the quantity of cerium required todecontaminate 1 m² of surface (S/V ratio=10) depending on the depth ofcorrosion required for both the conventional method using cerium aloneor the method disclosed in the present invention.

TABLE IV Comparison of quantities of cerium required to decontaminate 1m² of surface (S/V ratio = 10) depending on the depth of corrosionrequired Invention (HNO₃ 4 mol · 1⁻¹, Conventional method: Ce 10⁻² mol ·1⁻¹) Thickness/μm quantity of cerium/mol quantity of cerium/mol 1   0.51 2 1 1 3   1.5 1 4 2 1 5   2.5 1 6 3 1 8   3.5 1 4 4 1 9   4.5 1 10  51

In the conventional method the quantity of cerium required increaseswith the thickness of steel to be corroded.

The present invention gives, for example, a corrosion speed of 0.5μm.h⁻¹, or 10 μm in 24 hours, using a solution of 1.10⁻² mol.1⁻¹ ofcerium in nitric acid 4 mol.1⁻¹ with bubbled ozonized oxygen.

What is claimed is:
 1. Method for the decontamination of a steel surfaceconsisting in bringing the surface to be decontaminated into contactwith a pickling solution comprising nitric acid and a first oxidationagent of the metal constituents of the steel surface such that the faceof the said surface is eroded by the oxidation of the metallicconstituents it contains, wherein said bringing into contact is achievedby direct, continuous introduction into the pickling solution of a gascomprising a second oxidation agent to continuously homogenize andoxidize at least partly said first oxidation agent reduced by theoxidation of the metallic constituents of the surface, wherein thepickling solution is an aqueous solution containing nitric acid at aconcentration of approximately 0.5 to 5 mol.1⁻¹ and cerium nitrate at aconcentration of approximately 0.001 to 0.1 mol.1⁻¹, and wherein thecontact temperature is maintained in the range of approximately 10 to80° C.
 2. Method of claim 1 wherein the first oxidation agent is ceriumnitrate IV and the second oxidation agent is ozone.
 3. Method of claim 2wherein the gas including ozone may also include at least one gasselected from either oxygen and nitrogen.
 4. Method of claim 2 whereinthe gas including ozone includes approximately 1 to 20% ozone.
 5. Methodof claim 3 wherein the gas including ozone includes approximately 1 to20% ozone.
 6. Method of any of the foregoing claims 9-13 wherein thesteel surface is a surface of an internal circuit of a reprocessingplant for irradiated nuclear fuels.
 7. Device for the radioactivedecontamination of a steel surface of an internal circuit of areprocessing plant for irradiated nuclear fuels according to the methodof the claim said circuit being provided with an air-lift, a device inwhich the air-lift is used both as a means for introducing the gascontaining the second oxidizing agent into the pickling solutioncontaining the first oxidizing agent, and as a means for homogenizingthe pickling solution when the said solution is brought into contactwith the steel surface to be decontaminated.
 8. Device of claim 7 alsocomprising an assembly for producing the second oxidizing agent. 9.Device of claim 8 wherein the first oxidizing agent is cerium IV, theassembly for producing the second oxidizing agent being an ozoneproduction assembly.